(1) Field of the Invention
The present invention relates to a frame forwarding apparatus and, more particularly, to a frame forwarding apparatus that connects a plurality of subscriber terminals to an Internet Service Provider (ISP) network, utilizing Virtual LANs (VLANs).
(2) Description of Related Art
When building an ISP network for connecting a plurality of subscriber terminals to the Internet, ISPs tend to adopt a Layer 2 frame forwarding apparatus, which is referred to as a Layer 2 Switch (L2SW) in this specification, instead of an expensive Internet Protocol (IP) packet forwarding apparatus because the L2SW is less expensive. Since a network using the L2SWs works LAN-based, there is a possibility intrinsic to the LAN that transmission data, addressed to a particular user, are intercepted by any other user terminal belonging to the same LAN. For this reason, in ISP networks using L2SWs, Tag VLANs (Virtual Local Area Networks) in Ethernet are used to assign an individual VLAN to each subscriber or each particular user group in order to assure secrecy of data communication across the LAN.
As regards switching apparatus for relaying VLAN frames, a switching apparatus enabling routing of both a received frame including a VLAN tag and a received frame not including a VLAN tag by utilizing the correspondence of each IP subnet address with a VLAN identifier is described in, for example, Japanese Unexamined Patent Publication No. 2003-244185.
In the ISP network, there exists an IP packet forwarding apparatus (ISP router) to connect to the Internet. “VLAN Aggregation for IP address Allocation” of RFC 3069 Network Working Group discloses effective IP address space utilization for IP subnets to be allocated to each subscriber terminal, by adopting a scheme such that a plurality of subscribers' VLANs (sub-VLANs) belong to one super-VLAN and a router allocates a common gateway address to these subscribers' VLANs.